Gold nanoparticles produced using water droplets

Chemists at Stanford University have stumbled upon a method for producing gold nanoparticles using water droplets, the latest discovery in the growing field of ‘on-droplet’ chemistry.

According to the researchers, this discovery could lead the way to more environmentally friendly approaches to producing gold nanoparticles and other metallic nanoparticles.

“Being able to do reactions in water means you don’t have to worry about contamination. It’s green chemistry,” said Professor Richard Zare, a distinguished chemist based at Stanford University.

While gold has long been noted for its beauty and chemical stability on a macroscopic scale, more recently, gold has attracted interest due to its nanoscale properties: it is an excellent catalyst, highly reactive and has useful optical properties. Gold nanoparticles show promise in a range of applications, including in targeted cancer treatments.

Despite the usefulness of gold nanoparticles, the only reliable means of creating gold nanoparticles has been to combine chloroauric acid with a reducing agent to free gold atoms in the form of spheres, prisms, rods and wires, all of which have different properties.

Chemical reactions are often sped up when microdroplets are used and Zare and his colleagues found that using microdroplets of chloroauric acid with the reducing agent caused gold nanoparticles to grow more than 100,000 as with bulk acid.

The researchers discovered – to their surprise – that when the reducing agent was replaced with microdroplets of water, gold nanoparticles were still released. An electron microscope revealed the tiny nanoparticles and nanowires: fused with particles attached to the threads of the wires like “berry clusters on a branch”.

“Much to our bewilderment, we found that gold nanostructures could be made without any added reducing agents,” said Jae Kyoo Lee, a Stanford University research associate.

Their accidental discovery that microdroplets of water can serve as microreactors in the production of gold nanoparticles opens up the possibility that reactions in water droplets could be ‘fundamentally different’ to those in bulk water. If scaled up, the researchers suggest, this process could eliminate the need for polluting and potentially toxic reducing agents.

According to Zare, it is possible that mirodroplets of water served as a replacement reducing agent in this case due to the vast increase in surface area allowing a strong electric field to form and encourage the formation of gold nanoparticles.

“The surface area atop a one-litre beaker of water is less than one square metre. But if you turn the water in that beaker into microdroplets, you will get about 3,000 square metres of surface area: about the size of half a football field,” said Zare.

The team is now exploring approaches to use the gold nanoparticles and nanowires in chemical and biomedical applications, as well as to adjust their technique such that it could also be used to create thin gold films.